1. Field of the Invention
The invention relates to a demapper and method thereof, and more particularly, to a demapper utilizing a known less significant bits to estimate the corresponding more significant bits and method thereof.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a block diagram of a prior art 64-Quadrature Amplitude Modulation Trellis Coded Modulation (64-QAM-TCM) encoder 10. The 64-QAM-TCM encoder 10 includes a parser 20, a differential encoder 22, a convolutional encoder 24, a puncturing unit 26, and a QAM mapper 28. When the parser 20 receives four symbols RS1, RS2, RS3, RS4 in order, the parser 20 will disorder bits inside the symbols RS1, RS2, RS3, RS4, and then reorder them and output the reordering result; wherein the in-phase and quadrature-phase more significant bits in these symbols RS1, RS2, RS3, RS4 are directly input to the QAM mapper 28 from the parser 20 without being encoded, while the in-phase and quadrature-phase least significant bits in the symbols RS1, RS2, RS3, RS4 are input to the QAM mapper 28 after being processes via the differential encoder 22, the convolutional encoder 24 and the puncturing unit 26. The above-mentioned QAM-TCM decoding technique is familiar to those of average skill in the art, and is further detailed in the publication, “Digital Multi-Programme Systems for Television, Sound and Data Services for Cable Distribution,” ITU-T Recommendation J. 83. Therefore, further related descriptions are omitted for the sake of brevity.
The prior art has disclosed a QAM-TCM decoder for the purpose of processing the signal output from the above-mentioned QAM-TCM encoder; however, the prior art directly performs depuncturing and Viterbi decoding upon the in-phase and quadrature-phase bit streams X, Y received via a transmission channel. Consequently, the decoding processes that require a large number of calculations will thereby increase the complexity of the decoding operation. As to another prior art disclosure, a Viterbi decoder is utilized to decode the in-phase and quadrature-phase least significant bits of the received in-phase and quadrature-phase bit streams X, Y; and the remaining more significant bits are estimated through a referencing of Euclidean distances. However, estimation of each set (X, Y) requires determining the Euclidean distances between the set (X, Y) and a plurality of possible solutions. To achieve this estimation is costly, as it requires a great number of computations.